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PRINTED FROM the OXFORD RESEARCH ENCYCLOPEDIA, AFRICAN HISTORY (oxfordre.com/africanhistory). (c) Oxford University Press USA, 2019. All Rights Reserved. Personal use only; commercial use is strictly prohibited (for details see Privacy Policy and Legal Notice).

date: 22 February 2019

Archaeobotany: Methods

Summary and Keywords

Archaeobotany’s goals are to investigate the interactions between human societies and the plant world in the past from the botanical remains preserved in archaeological sites, including the environment people exploited and the foods they extracted from it. Archaeobotanical research in Africa has tended to be less widely practiced than in many other parts of the world, and systematic archaeobotanical sampling is still only incorporated into a minority of archaeological field projects in Africa. Nevertheless, there is potential for archaeobotany to contribute to a holistic understanding of Africa’s past. The general scope of archaeobotany is outlined before focusing on how typical archaeobotanical remains relate to agriculture and food production. A short overview on the practical side of collecting archaeobotanical samples is provided. Archaeobotany’s two general themes are discussed: hunter-gatherer subsistence and the origins of agriculture.

There are many different ways to approach the study of domestication, agricultural evolution, and culinary traditions. Because plant foods (cereals and vegetables) are among the most common foods in Africa—and indeed, the world—archaeobotany is a key approach to studying the history of alimentation and the transformation of the environment that came about through food production. Archaeobotany is an interdisciplinary field bringing botanical knowledge and methods to archaeological contexts and materials. Archaeobotany is concerned with the recovery of the fragmentary remains of plants from archaeological sites, their taxonomic identification, and their interpretation in terms of past human activities and the environment. Archaeobotany draws upon a wide range of adjunct information in arriving at interpretations, such as modern botanical and genetic studies of crops, ethnographic observations of plant processing, cooking and land use, and experimental work like observations on the destructive and distorting effects of charring on the preservation of plant parts. In North America, archaeobotany is more often called paleoethnobotany, emphasizing the importance of modern studies to better understand the past, that is, links to ethnobotany, whereas the term archaeobotany emphasizes the archaeological nature of the evidence.1 Plants from the past may be preserved in various ways and thus, there are several subfields of archaeobotany. A general division in archaeobotany is between micro-remains and macro-remains. While macro-remains can be seen with the naked eye, they are still small and require microscopes for study. Macro-remains mainly include the study of seeds (and related structures) and wood charcoal, which tend to be different specializations carried out by different specialists. The smallest seeds are around 0.2 mm but most macro-remains are >0.5 mm and are studied with a basic low-power binocular microscope and reflected light. Wood charcoal fragments smaller than 2 mm are unlikely to be identifiable and microscopes need to range between 40× and 200×. In contrast, micro-remains are so small (<0.1 mm) that they can only be seen under a microscope. Sampling of sediments or residues for micro-remains are processed in a laboratory using what are essentially chemistry methods, with extracted remains mounted on microscope slides and studied with transmitted light usually at higher magnifications (100×–200×). The most widely studied micro-remains from archaeological sites are phytoliths, while starch has seen growing application, especially from human dental calculus and residues removed from food processing tools. Pollen is studied in a similar way but is more usually collected offsite in natural sedimentary sequences, such as lake cores, to reflect regional and longer-term vegetation patterns rather than shorter-term, localized human activities.

Nevertheless, plant macro-remains provide the most direct evidence for understanding early crops, food, and agriculture. As archaeobotanical research expands in Africa, it is bringing a new body of data able to improve our knowledge of food and agricultural history.

The Scope of Archaeobotany: From Landscape to Food

Archaeobotany tends to address one of two themes, either the reconstruction of landscapes or the reconstruction of past food traditions. The study of agriculture, of course, can be regarded as including both of these themes and is more often than not the primary focus of archaeobotanical studies. In terms of landscape, archaeobotanical evidence provides a dataset, filtered by human activities of procurement, disposal, and preservation, that reflect the vegetation exploited by past communities. Wood charcoal studies in particular tend to address questions of vegetation around sites and how this changed over time.2 Seed assemblages, by contrast, more often represent the more limited ecologies of agricultural lands and selective acquisition of edible wild foods. In addition to issues around food and agriculture, it is worth flagging the fact that archaeobotanical research in Africa has included some leading-edge research on the reconstruction of past vegetation change from wood charcoal, human adaptation to those vegetation changes, and how some changes have possibly been driven by human activities.3

While archaeobotany identifies some key plant species people consumed in the past, food and drink studies are about more than just documenting the arrival of new practices or new crop taxa; they are about embedding such practices in the social, economic, political, and environmental history of a society. Indeed, procurement of food is a vital human concern, “the end point of that major activity of humankind (reproduction apart), that is, the production of food.”4 To reach an understanding of this “end point” of food consumption, it is important to understand how people produced food. Before the long processes of animal and plant domestication, the main human activities were the gathering of wild plants and animal hunting. Ever since the transition to agriculture, when societies fell into the domestication “trap,” however, people have been forced to grow grain and herd cattle for food in various combinations.5 These agropastoral activities required a number of phases in order to reach the point of food consumption (i.e., as a trajectory from procurement to consumption).6 This chaine opératoire of food includes primary production, harvesting and initial processing, storage, post-storage processing, cooking, consumption, and disposal. Each of these phases has varying potential for preservation in the archaeobotanical record and may be studied through different lines of evidence.7 For foodstuffs, production and consumption are inseparably linked. Food can be considered as passing through processing phases that filter a potential assemblage in terms of plant components, both taxa and plant parts. Archaeobotanically oriented ethnoarchaeology and experiments have helped to reveal how archaeobotanical assemblages can preserve some practices of crop processing and food preparation.8

From a botanical point of view, production processes concern the entire phase of primary production, from soil preparation through sowing and tending and harvesting from the fields. A range of potential human practices contributes to field conditions, from the nature of tillage (depth, extent of soil turn over), soil enrichment and manuring, soil wetness (through field placement or irrigation), to choices of mixed cropping. Many of these practices may be inferred from the ecology of preserved weed flora, working from the assumption that weeds in archaeobotanical assemblages typically derive from crop-processing waste and therefore reflect ecological conditions in the field.9 One challenge in applying these assemblages in Africa is that many have been small, providing non-robust datasets. Another challenge is that ecological inferences are built on reliable taxonomic identification, preferably to species, but sometimes possibly only at the genus level, and limited reference collections often do not allow for such taxonomic resolution with any confidence in Africa. Finally, many key African crops, notably pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor), tend to be harvested by cutting only the spike and thereby not incorporating weeds from the arable field. In recent years, preserved weed seeds have begun to be complemented by stable isotopes on grains, which may indicate manuring through elevated 15N isotopes and wetter conditions through elevated 13C isotopes.10 However, much of this development has focused on Mediterranean Europe and Western Asia and baseline studies on wheats and barley. Baseline studies are needed on African crops to take this approach forward in African contexts. In Asia, archaeological phytolith assemblages have been used to infer the ecology of arable systems, such as distinguishing wet versus dry rice (Oryza sp.).11 These approaches also have promise for the development of phytolith studies in Africa.

After harvesting, the crop product is filtered from contaminating weeds and nonedible plant parts by crop processing. After harvesting, the aim of crop processing is to remove the edible from the inedible, liberate the grain from the husk and chaff, and remove the weed seeds. Varying amounts of crop processing takes place before the crop is stored or distributed.12 As these pre-storage, post-harvest activities are likely to have been concentrated in time (once per year), they are likely to be underrepresented. Further, they are likely to take place in or near the fields, away from settlements and potential venues for charring, and as such they will be underrepresented in the archaeobotanical record. It is possible that waste from such events might be saved as fuel or fodder and make its way onto sites, but even so, waste will be relatively rare in charred assemblages. However, these bulk waste products may be preferred as additives to the seasonal production of chaff-tempered ceramics or mud brick. Thus, where potting traditions call for chaff tempering, this seasonal threshing and winnowing will likely be represented.13 By contrast, when crops are taken from storage for additional processing before cooking and consumption, it is likely to take place regularly (perhaps daily) in domestic contexts. This post-storage processing waste can be expected to take place hundreds of times more often than harvest-time processing as well as being in close proximity to routine charring conditions. Thus the most frequently encountered archaeobotanical assemblages are likely to represent routine post-storage processing, which in turn provide some insight into the state of crops when stored.14 This in turn may reflect the scale at which crops are processed or labor mobilized at the time of harvest.15

For non-mechanized farmers, such as those in Africa, there are few methods by which a crop may be processed. These methods involve a combination of pounding or threshing and separation by winnowing or sieving. In terms of assemblage composition, all these stages of crop processing imply some characteristic plant remains. In this sense, crop processing can be divided into two categories of activities: first, those actions applied to break apart the crop plant, such as threshing (cereal ears and pulses pods), and later dehusking, and second, those used for the separation of non-food elements from the broken crop elements. Separation of lightweight vegetal parts, such as chaff, can be done by winnowing, while sieving can be used to separate larger elements, such as weed seed heads from grain (coarse sieving) or grain from small weed seeds (fine sieving).16 Therefore, the different stages will produce waste with distinctive characteristics associated with the weight and size of various elements. It is notable that some African crops, such as rice, bicolor race sorghums, and fonio, are hulled, while others (e.g., pearl millet and the sorghum races caudatum, guinea, and durra, as well as finger millet [Eleusine coracana]) are free-threshing. The difference between husked and free-threshing plants will have an effect on both processing and the plant remains assemblage. The routine dehusking of hulled cereals can be expected to produce regular assemblages of grains, chaff, and weeds. By contrast, free-threshing cereals are likely to be stored as clean grain, leading to less routine loss of grain or chaff that might be preserved by charring.17 This may contribute to making some cereals, such as finger millet, rarer in the archaeological record.

Storage serves the fundamental purposes of conserving grain through the year before sowing in the next year, of sustenance until the next harvest, and of serving as a reserve for times of bad harvests. The process of storage has a strong effect on the composition of archaeobotanical remains because it separates the seasonal processing stages, those associated with harvesting and often carried out near the fields, from the daily routine post-storage processing.

For purposes of putting together a holistic archaeology of food production and consumption, the archaeobotanical evidence for production and preparation can be linked to the artifactual evidence for preparation and consumption, as well as inferences about storage. Figure 1 outlines a holistic archaeology of the food system, a chaine opératoire from field to food (Table 1). This holistic perspective combines evidence from artifacts, crop processing, and animal remains. Archaeobotanical analysis of crop production and storage provides information about the social system in terms of labor organization, while ceramic evidence may relate to changes in social contexts of consumption, which may play a role in differentiations within societies or between societies (ethnogenesis).18

To interpret the meaning of archaeobotanical remains, it is first necessary to comprehend the formation process of typical charred remains. In others words, one needs to apprehend what the composition of the flotation sample is, how its components arrived in the archaeological deposit, and what was destroyed and not deposited. As stated by Fuller, Stevens, and McClatchie: “To state the obvious, charred remains only become charred and preserved through virtue of having come into contact with fire. It is then surely impossible to interpret the relationship between context and charred plant remains without considering how both relate to the fire responsible for the assemblage's preservation.”19 While some charred remains will come from fire spots (e.g., from within the hearth where they were burned), in many, if not most cases charred remains are dispersed in other archaeological deposits and therefore distant from the context of the activities by which they became charred. Even if they are recovered from the context of charring, they may come into that context prior to charring from one or more distinct activities.

Because there is no straightforward relationship between archaeological contexts and the meaning of the plant remains in them, archaeobotanists have devised various approaches in order to understand this relationship. Hillman, for example, advocated the ethnoarchaeological study of crop-processing activities to infer how such processes might structure the quantitative content of assemblages.20 In terms of working from archaeological assemblages, it is useful to categorize these by context type, including how they relate to potential plant preservation. Three broad categories are recognized.21 The first category (A) is where the remains have burnt within the context from which they were recovered. The context will show signs of burning. In this case, remains can be considered as coming from a primary deposit.22 The second group of remains (B) come from a single burning event but the remains have been moved to another unburned context (secondary deposition). The third group (C) differs from group B by the way that the assemblage was formed, deriving not from one single event but from many different charring events mixed together during or after deposition. In other words, it is averaged across multiple episodes of behavior.

Contrary to what is often assumed in archaeology, that there is a direct link between activities that created archaeological assemblages and the excavated context from which they were recovered, an approach to patterns across group C samples provides insight into the most recurrent, prevalent, and practical routines. When present, the less common samples of group A or group B types can be compared to a “behavioral context” approach to interpretation that links plant processing, burning, and archaeobotanical evidence.23 However, such samples are less prevalent, and it is the interpretation of the content of samples without secure behavioral context that should be concerning. Fuller et al. (2014) argue this requires an ability “to break the tyranny of context for the power of content.” As the class C assemblage is the result of many different activities, random archaeobotanical sampling tends to produce a similar homogenized assemblage. This approach shows that regardless of context type, most contexts on a site have a similar assemblage, and this assemblage relates mostly to routine activities. Charred remains of group C samples represent the background noise of plant use in everyday refuse.24 In the case of agricultural societies, this noise tends to provide evidence for major crops, crop-processing waste, including arable weeds, and therefore an agricultural environment. Following this background noise approach, each sample can be considered to have its importance in revealing routine activities by site or time period: “Charred plant components resulting from routine activities that are conducted day in, day out are 365 times more likely to be represented than the once-in-year or occasional event. [. . .] The internal composition—by which we mean all those seeds/chaff from a single flotation sample—of the assemblage itself . . . is most informative.”25 Thus the study of one single flotation sample (by archaeological phase or even by site) may in itself be good enough to reflect a past behavioral pattern.

Another potential source of charred seed remains that is nonagricultural is through fuel use, especially the burning of dung fuel, which may incorporate seeds from the diet of animals. This source of charred seeds in Western and Central Asia has been much discussed.26 The extent to which dung fuel should be considered to dominate the archaeobotanical record has been a point of debate.27 To be sure, some seeds, especially those with hard seed coats (e.g., Chenopodiaceae, Cyperaceae, and various legumes), survive digestion and may enter hearths in this way.28 However, experiments have shown that clean cereal grains, the most common archaeobotanical remains on most agricultural sites, do not survive digestion by ungulates (such as sheep, goat, or cattle).29 Ethnoarchaeological investigations of dung-fuelled hearths in India found little seed evidence from the dung compared to the residues of crop processing.30 In addition, in regions with a damp climate, from the damp temperate zones of northern Europe to the wet tropics of central Africa, the charring of dung is improbable, as it would be difficult to dry to the point of being combustible.31 Some consideration of dung as a potential source of seeds is warranted, especially if a range of small, hard, wild seed types are present that seem improbable as agricultural weeds; sometimes charred fragments of dung are present, as noted, for example, from Iron Age samples from Northern Sudan.32 In better-studied cases in western Asia, it seems likely that some dung fuel-dominated assemblages occur alongside or even mixed with those deriving from routine food and crop waste.33 Some argumentation from assemblage composition is needed to make the case for dung, but even so, it may well be mixed with routine food-processing remains—the more likely source for quantities of food crop grains.

To conclude, archaeobotanical macro-remains, especially from typical grain-rich assemblages, provide evidence for routines of daily processing. They also have implications for how harvesting and storage might have been organized in terms of the scale of labor units. Signals of typical patterns are expected for particular sites or periods, and in the absence of multiple spatial samples across urban sites, most differences are expected to be diachronic. Other food plant uses (such as fruits and nuts), seed processing (such as cotton deseeding), and fuel sources, such as dung, may add complications to the patterns.

Field Sampling for Archaeobotanical Datasets: Overview

First, it should be kept in mind that there are various archaeobotanical subfields, but broadly these can be divided into two categories in terms of sampling strategies: macro-remains and micro-remains.

The first set, macro-remains or remains that are visible to the naked eye (>0.25 mm), is the more important set of archaeobotanical data.34 There are five groups of macro-remains: seeds (which normally include a range of fruits, seeds, chaff, and related fragments), wood charcoal (by far the most common category), parenchyma (charred fragments from roots, tubers, or large fruits), charred remains of cooked food, and impressions in ceramics of mud bricks. The study of the last category, impressions, really requires subsampling from ceramics or other clay materials if and when voids of plants are evident in the surface. Often, impressions represent only a minority of ceramic remains, and only a minority of those impressions present are likely to be identifiable. Nevertheless, those impressions may provide not only identifications of taxonomic presence but potentially preserve parts of plants relevant to domestication studies. For example, ceramic impressions of chaff have provided to date the best evidence for early domestication processes in pearl millet (Pennisetum glaucum) in West Africa and sorghum in Sudan.35 The main procedure for working is to assess impressions and then cast them with a silicon-based material so as to have a positive, which can then be photographed, normally at a higher resolution allowed by a scanning electron microscope (SEM).36 Recently, the use of CT scanning has begun to offer the potential for the virtual extraction of plan inclusions from within ceramics.37 For all other categories of macro-remains, flotation is the normal procedure (see Table 2).

Micro-remains are part of a broader range of potential environmental datasets studied from small sediment samples, including various forms of geoarchaeological sampling.38 Small bulk sediments are also needed from plant micro-remains, of which phytoliths are the most widely used. Phytoliths (plant silica) are disarticulated, nonspecific plant morphotypes. They are nonspecific in that most phytoliths represent small fragments of plant tissues rather than pieces of discrete whole organs such as seeds or pollen grains. Numerous phytolith forms are produced in a given plant and species, and there is extensive sharing of forms between different species (especially among grasses). While occasional morphotypes are more taxonomically diagnostic, especially when still articulated into multicell groups, many phytoliths are more characteristic of plant parts rather than plant species. It is this latter patterning that makes them useful for crop-processing studies and other investigations of spatial patterns in past plant use and deposition.39 On archaeological sites, samples for phytoliths and for various sedimentological analyses are collected and bagged from freshly exposed contexts or stratigraphic sections. They are bagged in bulk in relatively small quantities (on the order of 50–100 mL or 50 g, which is enough to allow for replicate subsampling) and processed in a chemical laboratory.40 Sampling procedures are the same for pollen, which tends to have little relevance to studies of food or agriculture, but may provide insights into regional environments. Pollen data are usually best when collected from natural stratigraphy (e.g., lakes) rather than the complex and disturbed stratigraphy of archaeological sites. The same samples can potentially be used for phytoliths, pollen, and sedimentological analyses (and sample sizes may need to be multiplied). Caution must be taken to minimize contamination by windborne particles and by the mixing of sediments. This necessitates an immediate collection after exposure.

Field Sampling: Flotation

Flotation uses buoyancy to separate materials of different density. In particular, plant remains, usually charred, are lighter than sand, gravel, pottery, or bone. As noted by Mark Nesbitt, “Flotation works on a simple principle: soil particles sink, charred plant remains float.”41 Whether mechanical (using a flotation drum or machine) or manual (using buckets), the basic principle is to mix the sediment with water to wet it and break it apart. The buoyant material—including the plant macro-remains—is then poured off and collected in a fine mesh.42 This process was pioneered by Hans Helbaek and Stuart Struever in the early 1960s and was used for the first time in Illinois (by Struever) and in the Deh Luran Plain in Iran (by Helbaek).43 In strict terms, “flotation” may be a misnomer since not all charred plant remains will actually float on the surface of water, but many do, and given sufficient time, most charred remains will become wet and slowly sink. The basic principle of flotation is that sediment is added to water and mixed so as to thoroughly break apart and wet the sediment; then the buoyant material is poured off and collected in a fine sieve (normally 0.5 mm or as small as 0.25 mm). In some variants, the floating material may be scooped off the surface of the water with a handheld sieve (such as a tea strainer), although methods that involve wash over (pouring) are more effective at retrieval of some of the slightly denser charred remains. The main division in flotation systems is between mechanical devices, where water is pumped in from below the sample, mixing it and agitating it, and the float then spills over a spout into a collecting sieve (often a series of nested sieves of varying sizes are used for convenience), and manual bucket flotation. Bucket flotation has the advantage of being highly portable and is often suitable for remote field contexts in Africa, including field surveys. Its main requirements are plenty of water—which may be a problem in some areas—and plenty of labor or time.

Figure 2: From the fields to the laboratory: an illustrated example of archaeobotanical chaine opératoire.

Photos by Louis Champion.

While variants exist, a simple wash-over method of bucket flotation is reliable and portable. In this method, the sample (or part of it, usually 3–5 liters of sediment) is placed in a bucket, water is added, and it is stirred up by hand; clods are gently broken down manually (Figure 2). The water is then further stirred and poured over the lip of the bucket into a receiving sieve (often 0.5 mm, although 0.25 mm may sometimes be preferred) where the float is caught. Sand and the heavy fraction remain in the bucket. A certain amount of skill is involved in pouring fast enough to coax out the denser charcoal, but not too fast so as to pour out fine sands that may clog the mesh. All of the charcoal does not normally come out in the first round of pouring, and therefore the process should be repeated by adding more water and stirring. The number of repetitions will vary with the quantity of plant remains and clays in the soil, but at least four or five repeats are the minimum. During the final repeat, the sediment should not be stirred again but the bucket can be gently rocked to wash over with the action of waves the last remaining seeds and charcoal. The heavy fraction that remains in the bucket is then emptied onto a coarser metal sieve (1 mm or 2 mm) and wet sieved for bones or artifacts. This whole process is repeated again with further sediment until the entire sample has been processed. A simplified procedure is outlined in Table 2.

Table 2. Archaeobotanical Sampling Instructions

• Ideally, each sample should consist of 10–30 liters (2–3 buckets) of soil.

• During the excavation, the excavator selects contexts as it samples and measures the progress of labor.

• Ash- or charcoal-rich contexts (fireplaces, pits) are sampled preferentially. For deep pits (over 1 m), two samples are preferable: one from the base and one from the upper part. Generally deep samples are best.

• If a context has less than 10 liters volume but seems rich (e.g., ashy), the volume is not problematic; a sample can be as little as 1 liter (or even less if needed).

• Do not try to assess whether seeds are visible in a sample—archaeobotanical macro-remains are in the order of halves of millimeters (between 100 microns and 2 mm) in size.

• The presence of modern seed (uncharred) is often due to a biodisturbances (via termites, ants, etc.) and does not correspond to archaeological evidence.

In brief:

1. Collect the sample during the excavation.

2. Preferentially sample contexts that are ashy such as charcoals, calcine fireplace, and pits.

3. Collect from 10 to 30 liters (may be smaller).

4. Pack in sample bags (no more than 10–15 liters per bag).

5. Enter the site, trench, context, and depth of the sample on a sticker and place it in the bag (e.g., BLAF-13-SX C19 100 cm).

6. Close the bag with a rope.

The Flotation Methodology

Once the soil sample is collected, flotation can begin. Flotation is a method widely used to separate materials of different density and is the most common way of recovering charred plant remains from archaeological contexts.

Bucket flotation is easiest and cheapest and also is a more convenient method for remote sites. Bucket flotation is very simple and can be learned quickly and easily. Apart from logistical benefits, bucket flotation is recommended for tropical sites (where preservation is often poor and sediments are very organic and clayey or sticky) because it provides better control over recovery than machine or drum systems.

Laboratory Methods

Identification and Quantification

The weight and volume of samples are normally recorded after flotation, which allows for estimates of total charcoal, although the presence of uncharred contaminants such as roots needs to be accounted for. Sorting normally proceeds at a low-powered microscope after splitting the sample into size fractions, which makes sorting more efficient (e.g., >4 mm, 2–4 mm, 1–2 mm, 0.5–1 mm, <0.5 mm). For all samples analyzed, whole vegetal items are recorded by count in a list of taxa, from which tables of relative frequency are constructed and ubiquity can be calculated. Relative frequency is the percentage of total seed count. In addition, ubiquity, which is the number of samples in which a taxon is present as a percentage of the total number of samples, is widely used. Ubiquity is a useful means of comparison of archaeobotanical datasets from different sites and laboratories, as it avoids the potential bias of extremely high or low frequencies of a taxon due to differential seed productively or preservation biases. Some researchers have argued that ubiquity (also called presence analysis) may be more appropriate for comparisons between sites and researchers, as it will reduce biases between the recovery and recording of different taxa counts.44 Ubiquity is also calculated across regions and periods in search of broader patterns in agriculture.

In brief, following this chaîne opératoire from the field to quantification, a catalogue that summarizes the counts of archaeobotanical finds by sample, archaeological context, phase, test unit, site, and area is obtained. The catalogue is a very good tool for making a quick analysis of stratigraphic change. The catalogue is the starting point of all required study. Aside from the fact that it offers a degree of transparency and makes it possible to reanalyze the data in the future, it also enables quick comparisons of different assemblages and identifies angles for subsequent study. Additionally, it provides a good means to compare different archaeological finds, such as ceramics, from the same site.

Research Theme: Hunter-Gatherer Plant Use

As the cradle of Homo sapiens, Africa has the longest archaeological record of hunter-gatherer societies, but these have been little documented from an archaeobotanical point of view. Ethnographic hunter-gatherers in Africa have provided many key models used by archaeologists for interpreting foraging patterns, such as work on Hadza helping to define the distinction between immediate and delayed-return systems, the latter having more importance for storage.45 or the recognition that plant food resources have tended to be more important calorically than hunted resources, despite a tendency to emphasize “man the hunter” in treatments of prehistory.46 Some archaeobotany from hunter-gathers sites indicates the use of wild tubers by Homo sapiens in southern Africa, based on finds of charred tubers (e.g., the corms of Hypoxis sp. [“African potato”] and oily fruits of Pappea capensis [“bushveld cherry”] in the Later Stone Age Bloomplaas cave site in South Africa). Based on parallels with such Holocene finds, Deacon argued that the fragmentary charred plant material in older sites, such as the South African Cape region, could represent similar foraging and processing of edible root tubers at Strathalan c. 30,000 bp, and even as far back as c. 100,000 bp at Klasies River Mouth.47 Nevertheless, few systematic studies, for example, of parenchyma tissues, have been carried out. Elsewhere in South Africa, the Sibudu cave site in KwaZulu-Natal has produced a good carbonized seed record between 38,000 and 72,000 bp. Out of 935 seeds, 30 taxa were identified. While a diversity of edible fruits were identified, the majority of seeds were sedge nutlets. Taxa came from both the tropical evergreen and deciduous woodland environments, but an increase in deciduous woodlands (i.e., drier conditions) had occurred from 48,000 bp. Not all or even most remains are likely to represent gathered foods, however. The majority of sedge nutlets are suggested to have come from bedding and matting that was carbonized by subsequent fires on the site.48

Better information is available on seeds found in Northern African cave sites, dating to the terminal Pleistocene and early Holocene. In this general region, a number of recent excavations that employed flotation have provided evidence for broad spectrum gathering of tree nuts and wild grasses, including Taforalt and Ifri Ouadadane in northern Morocco, El Mekta in Tunisia occupied during both the Capsian period (10,000–7500 bp), and Haua Fteah in northeastern Libya.49 At these sites, pine nuts (Pinus halepensis) and acorns (Quercus sp.) were major foods of the terminal Pleistocene and early Holocene, as well as rhizome tubers from feathergrass (Stipa tenacissima).50 Some gathering of wild oats (Avena sp.) is also reported from Taforalt. Interestingly, the earlier Middle Stone Age levels at Taforalt (40,000–20,000 bp) provide more evidence for wild legumes (Lathyrus/Vicia) alongside Stipa tubers, suggesting that nuts may have been a later specialization or shift due to environmental change.51 The use of grass tubers here offers some parallels to the heavy reliance on sedge tubers in the Late Pleistocene Nile valley, evident at Wadi Kubbaniya, Egypt, c. 16,000 bp.52 And, in general, the use of monocot tubers (from sedges and grasses) has been hypothesized to represent an early human foraging adaptation.53 What these recent data highlight is that systematic archaeobotanical sampling has much to reveal about Pleistocene hunter-gatherer diets, their diversity, and their regional adaptations.

Research Theme: Early African Crops

Over the course of the Holocene, especially during the middle and later Holocene (from 6000 to 2000 years ago), much of Africa converted to agricultural production. Africa only came to be considered a region of agricultural origins since 1950 but research into this transition has tended to lag behind work on other continents. Although the Russian crop geneticist Vavilov, as early as 1926, included Ethiopia (Abyssinia) as a center of crop diversity and local domestication, it was not until the 1950s that more anthropologists and botanists began to look more widely at indigenous African agricultural origins. For example, Portères distinguished “primary agricultural cradles,” centers of genetic diversification that became foci of agricultural civilizations, including western Africa.54 The American anthropologist, Murdock, argued in 1959 that West African agriculture was distinctive and should be added to the list of centers of crop origins.55 Harlan, a pioneer in the study of crop wild progenitors in Africa, developed a concept of agricultural “non-centers” applicable to Africa.56 For him, this meant that a restricted geographical area of multiple crop origins could not be defined, but instead wild progenitors had use over much wider zones, leading to diffuse origins: “And even today, African agriculture is still often a mosaic of crops, traditions and techniques that do not reveal a centre, a core, or a single place of origin.”57 However, as has become clear even in so-called focused centers of origin, such as the West Asian Fertile Crescent, the origin of crops was a mosaic of different crops in different subregions, and evolutionary processes of domestication were slow processes of selection from the wild, with ample time for gene flow and movement of early crops geographically. Therefore, Harlan’s notional contrast between a center and a non-center is less clear-cut. In addition, it is suggested that the domestication of key African species was focused in restricted parts of their wild ranges and that subregional packages of domesticates can be defined. On this basis, Fuller and Hildebrand outline five regional centers of crop origins in Africa (Map 1), with at least four of these recognized as distinct centers of domestication in global syntheses.58 Each of these areas has specific staple crops, but details about the domestication processes for many of them are still lacking. There is now good evidence for the domestication and dispersal of pearl millet and sorghum, which is reviewed here, with some comments on current problems surrounding African rice and fonio domestications (Map 1).

Pearl millet is the common English name of the main cultivated millet of West Africa (Pennisetum glaucum), which is important throughout the savannas of Africa as well as those in South Asia. In the early 21st century, it is the staple food crop for over 100 million people in tropical Africa and India and is grown mostly in the arid and semiarid tropical regions of Africa and Asia. Pearl millet was domesticated in West Africa sometime before 2000 bc (Map 1). The archaeobotanical evidence is most often in the form of charred seeds and impressions in pottery, followed by involucre bases, rachis and bristle fragments, and spikelets. Charred grains can indicate domestication through a characteristic domestication change toward fatter club-shaped grains, while chaff such as those preserved as impressions in pottery can indicate change toward loss of natural seed dispersal and the development of multigrain involucres.59 The earliest archaeobotanical evidence for pearl millet comes from a group of sites located in the Lower Tilemsi Valley, Mali, which date from between 2500 and 2000 bc.60 The domestication changes already evident in this material (club-shaped grains at 2500 bc, nonshattering and multiseed involucres by 2000 bc) suggest that cultivation and domestication processes must have begun earlier, perhaps a millennium earlier.

Early agriculture in western Africa was focused on pearl millet alongside domesticated livestock. Pearl millet spread rapidly east across the Sahel and even over the sea to reach India earlier than 3700 years ago.61 In western Africa pearl millet spread gradually, reaching southern Ghana and the Lake Chad region by c. 1500 bc.62 It was during this period of initial dispersal that cowpea (Vigna unguiculata) appears to have been domesticated as an addition to agriculture, evident from finds in Ghana from 1700 to 1500 bc (Map 1, Figure 3).63 Oil palm cultivation and forest management may have also become established around this time in the more tropical and forested south of Ghana.64 Subsequently, these crops—pearl millet, cowpea, and oil palm—penetrated the rainforest zone in southern Cameroon by around 400 bc.65 Pearl millet along with sorghum is among the earliest crops attested to in East Africa, but attested to only in the first centuries ad at Kabusanze in Rwanda.66 These two cereals subsequently became widely established in agriculture throughout eastern and southern Africa.67

Sorghum bicolor seems to have been domesticated for the first time in an area bordering the eastern Sahara (Map 1, Figure 3).68 Recent evidence from ceramic impressions in eastern Sudan has documented the presence of a large proportion of domesticated-type non-shattering spikelet (i.e., a loss of natural seed dispersal) dating from 3500 to 3000 bc at the site of Kasm al-Girba 23.69 This evidence is in contrast to apparently all wild sorghum spikelet remains in Neolithic ceramic impressions from the Khartoum area dating from 5000 to 4000 bc, and the entirely wild-type assemblages from Early Holocene Saharan sites, such as Nabta Playa in Egypt, c. 7500 bc, or Takrakori in southwest Libya, 7300–6400 bc.70 After 3000 bc, archaeobotanical evidence is limited but sorghum populations cultivated near Kassala, Sudan at c. 1800 bc still show a mixture of wild and domesticated morphology.71 Sorghum became widely grown throughout the Nubian Nile valley from Meroitic times onward (i.e., from c. 400 bc).72

The original race of sorghum is race bicolor, with tight-clasping hulls, which require dehusking. It was this form that spread first throughout Africa and across the seas to India, perhaps as early as 4000 years ago.73 Other races (e.g., caudatum, guinea, and durra) evolved later and in parallel developed free-threshing forms and forms with larger grains and denser ears, making for more productive forms and forms requiring fewer labor-intensive crop-processing steps.74 The presence in India of the races, bicolor, caudatum, and guinea, is due to at least three separate introductions from Africa.75 The earliest sorghum in West Africa comes from Bénin more than 2500 years ago, and guinea probably evolved in the region later. Free-threshing caudatum may have evolved in the Sudan region and spread through Saharan oases or the Sahel around 2000 years ago.76 In eastern Africa, both bicolor and caudatum may have arrived together in the Iron Age, with little evidence for sorghum older than about 2000 bp.77 While other crops are inferred to have been domesticated in eastern Africa, such as Sudan or Ethiopia, including, for example, finger millet (Eleusine coracana), hyacinth bean (Lablab purpureus), and tef (Eragrostis tef), archaeological evidence is lacking for their origins and domestication process, although tef was apparently domesticated prior to finds in Eritrea and northern Ethiopia of c. 500 bc (Map 1, Figure 3).78

African rice, Oryza glaberrima, is thought to have been domesticated from a wild progenitor, Oryza barthii, around 3500–2500 years ago. Portères placed the original domestication in the Inland Niger Delta of Mali (Map 1) while modern genetic geography points to original diversification around the headwaters of the Niger River, where coastal and upland guinea rices split off from the main Niger basin group of rice.79 As of the early 21st century, with the exception of the site of Juffure, dated to cal. ad 1650–1900 in Gambia, all the sites with domesticated African rice come from the Niger Valley, so the antiquity of the western group of rice is problematic.80 The domestication of African rice (Oryza glaberrima Steud.) also remains problematic, with the earliest evidence for domesticated rice in the Iron Age before 400 bc at Dia in the Inland Niger area.81 Other evidence for domesticated rice is more recent, but it had spread down the Niger River as far as Bénin by ad 300 (Map 1, Figure 3).82

Other crops of West African origin are less well understood. For example, fonio (Digitaria exilis) is a widespread small-grained cereal (Figure 3), often producing a crop on marginal soils and with poor rainfall, but its origins remain obscure. Some have argued that its wide patchy distribution implies early domestication and spread before other crops.83 However, as of the early 21st century, archaeological evidence is entirely from the past 2000 years.84 The oldest archaeological evidence of fonio was found at the site of Janruwa C in Nigeria. This site is related to the Nok culture and fonio grains were directly dated to cal. ad 100–350.85 Fonio was also found at Cubalel Phase II in Senegal, dating to ad 400–600 (Map 1).86 Such data tend to support the inference of Portères that it was domesticated in the Inland Niger Delta during the last millennium bc.87 If rice represents agricultural diversification into more productive wet habitats, fonio can be suggested to be a parallel diversification in cropping focused on the more marginal, drought-prone sites.

African agriculture, however, does not rely on endemic crops only, but has also incorporated major crops originating in Asia and the Americas. Historical sources are quite revealing about the uptake of American crops, such as maize and cassava, over the past ~500 years, but the earlier introduction of tropical Asian crops was prehistoric and requires archaeobotanical approaches.88 In terms of geographical origins, timing and routes of introduction of Asian crops can be divided into three groups: those from Southwest Asia, which have mainly contributed to agriculture in North Africa, Egypt, Nubia, and Ethiopia; those from India and China that arrived via Arabia (Chinese millets, sesame, and tree cotton); and those from the Southeast Asian tropics, which have had their largest impact in central and eastern Africa.89 The latter crops included banana and plantain (Musa sapientum), cocoyam (Colocasia esculenta), and greater yam (Dioscorea alata), many of which are major traditional staples in parts of Africa. This tropical trio of crops has received attention and been debated, but still little in the way of hard evidence has been offered.90 A major challenge is that these are not plants reproduced by seed or identified by seed archaeologically. Instead, approaches such as the study of charred parenchyma tissue need to be developed in Africa.91 Some plant micro-remains (starch, phytoliths) may also help to trace these crops. Phytoliths of apparent banana have been reported from Cameroon as old as 500–400 bc, but the lack of other evidence means that the timing and route of arrival and transmission across Africa of this plant remains unknown and controversial.92 In general, these arrivals from tropical Asia connect to discussions of globalization processes around the Indian Ocean World, which are best documented in eastern and southern Africa starting from c.ad 750, at which point some Asian grain crops, including Asian rice (Oryza sativa) and mung bean (Vigna radiata), became crops on various East African islands around the same time that Asian (Austronesian) cultural traditions (and people) established agriculture in Madagascar.93

Clearly there is still much to be learned about the early history of African agriculture, and the wider application of archaeobotanical methods is therefore essential.

Discussion of the Literature

The history of research on this topic draws on three traditions of scholarship: botany, historical linguistics and comparative ethnography, and archaeobotany. The foundations of work on early African farming were laid by botanists, including Vavilov, Porteres, and Harlan, who documented the distribution of traditional crop diversity and wild relatives.94 Such work was essential for framing the likely geography of agricultural origins in Africa and its subsequent spread. Modern genetic research can be regarded as building on this work and refining the botanical evidence for crop origins and human translocation of plants around the African continent. This line of evidence was synthesized and brought together with ethnographic and archaeological evidence in a landmark volume edited by Harlan et al. (1976).95

Comparative ethnography and historical linguistics have made major contributions to the study of past African plant use. Murdock’s overview of African ethnography and cropping traditions was pioneering in this regard, and various studies from the 2000s by Ehret, Blench, and Boeston can be taken to represent the state of this research tradition.96 Most importantly, such lines of evidence highlight the historical relationships between modern traditions in different regions across the African continent. Putting actual dates on the processes of diffusion or migration that account for these links is more problematic without the empirical evidence provided by archaeological plant remains and subsistence technologies.

Archaeobotanical evidence has strengths in providing empirical evidence on the utilization of particular plants in time and space. Archaeobotanical remains have the potential to be directly dated by radiocarbon and can be related to past cultural traditions that may be related by continuities into the ethnographic present, as documented by comparative ethnography and linguistics, or also represent lost cultural traditions of the past. Much of the literature in African archaeobotany is quite recent, and its history can be accessed by a tradition of edited volumes arising from the International Workgroup for African Archaeobotany (see “Further Reading”).

(29.)
M. Wallace and M. Charles, “What Goes in Does Not Always Come Out: The Impact of the Ruminant Digestive System of Sheep on Plant Material, and Its Importance for the Interpretation of Dung-Derived Archaeobotanical Assemblages,” Environmental Archaeology 18, no. 1 (2013): 18–30; and Valamoti et al., “Prehistoric Cereal Foods.”

(88.)
J. C. McCann, Maize and Grace: Africa’s Encounter with a New World Crop, 1500–2000 (Cambridge, MA: Harvard University Press, 2005); and J. A. Carney and R. N. Rosomoff, In the Shadow of Slavery: Africa’s Botanical Legacy in the Atlantic World (Berkeley, CA: University of California Press, 2011).

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